1. Field of the Invention
The present invention relates to a thin film transistor, a display device having the thin film transistor at least in a pixel portion, and methods for manufacturing the same.
2. Description of the Related Art
In recent years, technology for forming thin film transistors (TFTs) using a thin semiconductor film (with a thickness of from several tens of nanometers to several hundreds of nanometers, approximately) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electrooptic devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
As a switching element in an image display device, a thin film transistor including an amorphous semiconductor film, a thin film transistor including a polycrystalline semiconductor film that includes crystal grains with a diameter of 100 nm or more, or the like is used. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is processed into a linear shape with an optical system, and an amorphous semiconductor film is scanned and irradiated with the linear beam, thereby being crystallized.
As a switching element in an image display device, further, a thin film transistor including a microcrystalline silicon film or a microcrystalline silicon film including germanium is used (see Reference 1: Japanese Published Patent Application No. H4-242724; and Reference 2: Japanese Published Patent Application No. 2005-49832).